Acoustic Properties of Organic/Inorganic Composite Aerogels
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1188-LL07-02
Acoustic Properties of Organic/Inorganic Composite Aerogels Winny Donga, Tanya Faltensa, Michael Pantellb, Diana Simonc, Travis Thompsond, and Wayland Donge California Polytechnic University, Pomona, CA, USA
e
a
Chemical and Materials Engineering
b
Physics
c
Industrial Chemistry
d
Mechanical Engineering
Veneklasen Associates, Santa Monica, CA, USA
Abstract Composite aerogels (with varying concentrations of silica and poly-dimethylsiloxane) were developed and their acoustic absorption coefficient as a function of composition and average pores size have been measured. The polydimethylsiloxane modified the ceramic structure of the silica aerogels, decreasing the material’s rigidity while maintaining the high porosity of the aerogel structure. The composite aerogels were found to exhibit different modes of acoustic absorption than that of typical porous absorbers such as fiberglass.
At some
frequencies, the composite aerogels had 40% higher absorption than that of commercial fiberglass. Physical data show that these materials have a large surface area (> 400 m2/g) and varying pore sizes (d ~ 5 - 20 nm).
Introduction When sound encounters a material, the pressure wave can be reflected, transmitted, or absorbed. Material properties such as elastic modulus and porosity directly influence how a material will interact with incident acoustic waves. For materials with a high elastic modulus, regardless of porosity, acoustic reflection is high.
Materials with both a low modulus of
elasticity and low porosity allow for acoustic transmission.
In terms of absorption, the
combination of low modulus and high porosity are ideal. (Table 1)
Table 1. Influence of elastic modulus and porosity on a material’s acoustic response. Modulus
Porosity
Example
Reflection
Transmission
Absorption
↑
↑
Concrete
High
Low
None
↑
↓
Steel
High
None
None
↓
↓
Water
Low
High
Low
↓
↑
Snow
Low
Low
High
In optimizing materials for architectural acoustic absorption, typically a high absorption coefficient across a wide range of frequencies is desired. Commercially available acoustic materials generally have high absorption at frequencies above 1000 Hz and negligible absorption below 800 Hz. The primary goal of this project is to develop materials with tunable modulus and porosity to prepare an adjustable acoustic absorber, specifically one that can absorb well below 800 Hz. A composite of silica aerogels and poly-dimethylsiloxane (PDMS) combines the low modulus and high porosity.
Silica aerogels have very high surface areas (300 – 1000 m2/g) and high porosity (80 – 99%) and can be synthesized through a liquid-precursor, room-temperature method. The high porosity leads to extremely low sound velocity (down to 90 m/s) through the silica aerogel. [1] However, the high elastic modulus also results in high reflection coefficients, especially at low frequencies. In order to modify the elastic modulus of the silica aerogels, PDMS is incorporated into the silica structure making Organically Modified Sili
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